Sulfonated poly(arylene ether sulfone) copolymers derived from sulfonated 4,4'-dichlorodiphenyl sulfone (S-DCDPS), 4,4'-dichlorodiphenyl sulfone (DCDPS) and phenolphthalin (PP) were evaluated as polymer electrolyte membranes for direct methanol fuel cells. The change in ion exchange capacity, state of water content, proton conductivity, and methanol permeability with S-DCDPS content are discussed. The proton conductivity increased linearly from 5.1 x 10(-3) to 4.7 x 10(-2) S cm(-1) at 30 degrees C and 95% RH with increasing sulfonation level. However, the methanol permeability increased linearly up to 20 mol% sulfonated monomer content, but then a sudden increase was observed, referred to as the "percolation threshold", at sulfonated monomer content > 20 mol%. The proton conductivity of the polymer membranes depended on the bound water and free water in the membrane, but their methanol permeability depended on the free water content. The maximum selectivity (i.e. proton conductivity/methanol permeability) of the membranes was obtained near the percolation concentration. The behavior of the proton and methanol transport mechanism is discussed in relation to the state of water in the membranes. (c) 2005 Elsevier B.V. All rights reserved.